1. Field of the Invention
The present invention is generally directed to ducted fan propulsion engines for aircrafts.
2. Description of Related Art
Vertical takeoff and landing (VTOL) is a highly-sought capability in aeronautics. The ability to ascend vertically bypasses the need for large horizontal surface areas from which to launch or land an aircraft. Unfortunately, the design needed to ascend and descend vertically does not comport well with the design needed for high speed forward flight. Helicopters offer efficient hovering but are encumbered by severe drag in high speed forward flight. Propeller driven or jet aircrafts minimize their drag to enable high speed forward flight and rely on their fixed wings for lift, which of course offer no VTOL capability.
Ducted propeller technology has the potential to offer both VTOL capability and high speed forward flight. Historically, attempts to develop VTOL aircraft with high speed capability (>200 knots) have repeatedly encountered difficulty in meeting speed, range, and payload requirements owing to excessive power and/or fuel requirements, high weight-empty fraction, or poor cruise efficiency. The advancement of electrical power storage systems and research into drone technology has increased the interest in ducted fans due to their safety, efficiency, and cost.
Ducted fans can have significantly greater static thrust when compared to an open propeller of the same diameter and power loading. For a ducted fan, operating statically, net pressures on the duct inner surface will contribute to thrust if the inlet area is larger than the outlet area (positive camber).
The principal drawback to the use of ducted fans is the significant drag that occurs at higher speeds due to the increased surface area. When the ducts are aligned perpendicular to the direction of travel and the ducts are used for uplift, the need to turn the air 90 degrees also incurs a high momentum drag penalty.
If is for this reason that most research has focused on preserving the design features which enables enhanced vertical thrust and minimizing the drag penalty in forward flight. This has caused the field to mostly overlook another attribute of ducted fans—the ability to vector thrust. Whereas the wake from an open propeller is largely uncontrolled, ducted fans have the ability to direct their wake so as to prevent downstream interference. This means that ducted fans have the ability to be used in unique design series. This invention retains the vertical thrust capability of ducted fans and significantly reduces the drag problem normally associated with ducted fans in forward flight.
Although progress has been made with respect to flight and in particular to vertical takeoff and landing aircraft, considerable shortcomings remain. The present invention seeks to leverage the advantage of ducted fans while minimizing their principal drawback by engineering the duct form to be used in series so as to reduce drag.
The invention enables rotors to be added in a series without significant increase of drag. This allows for aircraft to retain the high disk area needed for a high vertical takeoff and landing force and more efficient hovering.
In addition, performing suction along the longitudinal axis of an aircraft assists in maintaining laminar flow across the fuselage or wing and reducing drag. This boundary layer ingestion will lead to high propulsive efficiency by re-energizing the slower moving turbulent wake from an aircraft's fuselage.